Sorry to hear this Gandah. As mentioned earlier by zolkis it does sound as if the something happened to the membrane during "vacuuming".
It cant hurt to remove the cup and give it a visual, otherwise I see a trip to Tokyo in their future. On the bright side of things Foster Japan is very quick and thorough especially in communicating with you and keeping you updated.
After all is said and done if they require work you may be without them for 2 weeks max.
Cant hurt to email them with questions
support@kotori.fostex.jp they will follow up with you most likely within 24 hours or less. Best of luck in getting it sorted out.